We can't as that would cross a red line and give Russia an excuse to attack NATO, which as far as I understand is a defensive alliance. We can't get our military invovled but we can arm and train Ukraine.
We can only do so much, but what we NEED to be doing is boosting our own defensive capability to deter Putin from even thinking about attacking you.
But I suspect you already know that,
My analogy is like this.
Imagine the school bully hitting you weekly for a whole academic year, do you spend the summer collecting feathers to hit back, doing nothing or Learning self defense or some for of martial arts, so when the bully tries to hit you, not only are you ready but defend your self and send a clear message you can and will do so.
What can do at the moment is also patrol the borders and scrambke quickly should an enemy unit attempt to come near to the border or encroach on our territory, I am trying to be careful here as it is not just with planes.